The present invention by incorporating temporal thermal imaging using dual band infrared (IR) wavelengths enables locating underground objects such as subsurface explosive devices, underground tunnels and caves, and other subsurface structures and sites such as underground pipes or other objects, in addition to locating structural flaws such as cracks and corrosion in structures such as pipes, bridge decks or other objects whether buried or on the surface.
The temperature of the earth's surface is impacted by the many processes that control the net flow of energy into and out of its surface. The largest contribution is the direct heating by the sun. The surface can also be heated or cooled by conduction (flow of energy from warmer areas to cooler areas), convection (flow of energy due to movement of air/water above the surface) and evaporative cooling (flow of energy due to evaporation of water at the surface.) Each process has its own level of impact and timescale on which it is relevant.
The diurnal cycle is a primary driver (directly and indirectly) of the temperature of the surface. When light from the sun falls on the earth's surface the absorbed energy (the percentage of which is the surface albedo) will increase the temperature of that surface. When the temperature at the surface is higher than that below that surface, energy will be conducted downward. The amount of energy that the subsurface can absorb and the speed at which it can absorb it goes a long way toward determining the surface temperature.
A column of solid earth material above and including a hollow or semi-empty underground object has less thermal inertia (resistance to temperature change) than an adjacent, equal-volume column of solid earth material without the hollow object. Typical subsurface objects displace the host materials which surround them. A column of earth material above hollow, or partially-empty objects undergoes larger diurnal or seasonal temperature changes, has warmer than ambient surface temperatures at midday, or during autumn, and has cooler than ambient temperatures at predawn, or during spring. This applies to, e.g., subsurface explosive devices, tunnels, caves, drains, tombs, pipelines, channels, cisterns, sewers, vessels, bunkers, trailers, and other such structures and sites, as well as surface and buried structures having flaws such as cracks and areas of corrosion.
Scientists have routinely used long wavelength IR bands at 8-12 microns or medium wavelength IR bands at 3-5 microns for thermal imaging in limited applications. The single IR band apparent thermal image has a non-thermal spectral reflectance component which is useful, for example, rock type mapping, mineral recognition, or monitoring distressed crops. However, the conventional single band thermal imaging is difficult to interpret. It yields imprecise information that is insensitive to the subtle heat flow anomalies produced by subsurface flaws and foreign objects. These conventional single band imaging techniques fail to distinguish between surface emissivity clutter and true temperatures. In addition, emissivity-related noise, typically 1 or 2 degree ° C., cannot be removed using a single passive thermal IR band even when used in conjunction with another active laser reflectance IR band.
The more recent technique of Dual Band Infrared (DBIR) Imaging has numerous advantages over the conventional thermal imaging which utilizes only a single IR band. DBIR imaging has been used to detect buried land mines by exploiting temperature differences between the mine site and the surrounding soil. U.S. Pat. No. 4,005,289 describes this method, the disclosure of which is incorporated herein by reference. See also N. K. Del Grande et al., “Buried Object Remote Detection Technology For Law Enforcement, in Surveillance Technologies, SPIE 1479, p. 335, 1991,” which notes the difficulty of removing clutter from corrected temperature maps lacking thermal inertia diagnostics; N. K. Del Grande, P. F. Durbin, M. R. Gorvad, D. E. Perkins, G. A. Clark, J. E. Hernandez and R. J. Sherwood, “Dual-band Infrared Capabilities for Imaging Buried Object Sites”, in Proc. of SPIE Conference 1942; and Underground and Obscured Object Imaging and Detection, Ed. N. Del Grande, I. Cindrich and P. Johnson, Orlando Fla., pp. 166-177, Apr. 15-16, 1993, the disclosures of which are incorporated herein by reference.
The DBIR imaging technique reduces false detections produced by clutter. It decouples the heat patterns (associated with underground hollow or semi-empty tunnel sites) from the surface emissivity patterns (associated with clutter). Clutter typically produces 1 or 2° C. apparent temperature-difference patterns. Most sites with clutter cannot be distinguished from subsurface explosive devices or underground tunnels and caves using a single passive thermal IR band. The DBIR technique uses two passive thermal IR bands to separate the image's thermal components from its emissivity components.
The DBIR approach clarifies thermal emission imagery by combining images from filtered medium wavelength 3-5 micron (MWIR) (e.g., 4.3-5.1 microns) and filtered long wavelength 8-12 micron (LWIR) (e.g., 9.6-11.6 microns) focal plane arrays. It senses temporal heat flows from variable-depth objects and voids such as cracks (contained air gaps or corrosion). It does this at least two times during the diurnal or annual cycle, when solar-heated hollow or semi-empty objects produce above ambient or below ambient temporal thermal and thermal inertia signatures unlike those of foreign-object clutter or those of the undisturbed host materials.
The DBIR images have similar thermal emission patterns but different spectral reflectance patterns. This approach, unlike the long wavelength infrared or medium wavelength infrared single-band approach, allows the user to identify weak heat flows from underground objects such as subsurface explosive devices (SSEDs) or deep underground tunnels (UGTs) and caves apart from natural terrain and foreign object clutter.
Thermal inertia diagnostics have also been used in conjunction with DBIR imaging to map flaws in heated structures (delamination gaps in bridge decks and corrosion gaps in aircraft) by exploiting thermal differences between the flaw and the structural material. U.S. Pat. No. 5,444,241, incorporated herein by reference, describes this method.
The most recent technique is the Dual Infra-Red Effusivity Computed Tomography method: (DIRECT). This method adapts DBIR imaging of emissivity corrected temperatures for locating SSEDs and UGTs as described in U.S. Pat. No. 7,157,714, the disclosure of which is incorporated herein by reference. Surface heat flow signatures occur periodically during the day and night, for objects (e.g., land mines) less than 1 meter deep, and annually during diverse seasons, for hollow or semi-empty objects (e.g., drainage channels) more than 1 meter deep. See N. K. Del Grande, “Thermal inertia contrast detection of subsurface structures”, Proc. of the SPIE Conference, Thermosense XXXI, Vol. 7299, pp. 166-178, Orlando, Fla., United States, 14-15 Apr. 2009, the disclosure of which is incorporated herein by reference.
The DIRECT approach discussed in U.S. Pat. No. 7,157,714 uses a temperature simulation model (e.g., an Annual Surface Climate Energy Budget, ASCEB, Model) to locate temporal heat flows from underground objects at times commensurate with their depth, density, volume, material and effusivity compared to that of the ambient host materials. The ASCEB model inputs over a dozen environmental parameters to determine suitable thermal survey times and conditions for simulating soil temperatures and temperature spreads which will detect, locate and characterize the subsurface object or structure.
Optimum times for thermal imaging surveys depend on the object depth, dimensions, material, and contrasting host material properties. In most cases, some of these characteristics will be unknown initially. Initial estimates used to establish thermal survey dates and times are based on the daily (or annual) air temperature response and the undisturbed terrain temporal thermal properties. This provides input to a daily (or annual) surface temperature simulation model. The model simulates the surface temperature response times which enhance detection of underground objects from an airborne platform.
The DIRECT procedure records air and surface material (such as soil, roadway or water) temperature highs, lows, and high minus low temperature differences. The method highlights thermal and thermal inertia contrast at the borders of variable-depth objects having physical, thermal and temporal properties which differ from natural terrain. It designates suitable survey times based on the model to locate objects with variable depths, dimensions and host material properties. It collects thermal data at an alternative site with similar surface cover and host materials at times which provide optimum thermal and thermal inertia contrast.
The present invention provides improved temporal methods to the DIRECT approach by using an empirical approach to locate temporal heat flows from objects at survey times commensurate with their depth, density, volume, material and effusivity compared to that of the ambient host material. The approach of the present invention avoids the need to use temperature simulation models to locate temporal heat flows.